Magneto-optical (MO) recording media are formed by sputtering a thin layer or film composed of rare earth elements and transition metals in a desired composition onto a substrate such as a glass or plastic wafer or disc. Such MO recording media have been used for data storage and in audio reproduction applications to produce recordable compact discs (CD's). Targets used in the sputtering process are known to be made from a variety of materials and produced by a variety of methods. Various target materials and manufacturing processes can be found in U.S. Pat. Nos. 4,957,549; 4,620,872; 4,946,501; 4,915,738; and 4,992,095.
The desirability of one such sputter target material or manufacturing process over others usually depends upon the performance characteristics of the target in producing (i.e., sputtering) the layer of magneto-optical recording medium. The performance of each target is typically measured by the resulting properties of the sputtered MO film produced. These properties may include coercivity (Hc), coercivity uniformity, write (external field) bias sensitivity, carrier (signal) to noise ratio (C/N ratio), and block error rate. The desired properties of the MO film are likely to change from one application to another. For example, a MO film may exhibit a coercivity (Hc) level which is too high or too low for some applications and just right for other applications. While the coercivity desired for a given MO film may vary, in general, it is desirable for the coercivity to be kept high and the coercivity uniformity kept low. In addition, it is generally desirable for a given MO film to exhibit a high C/N ratio and to maintain a desired block error rate at as low a read laser power level as possible. It is also desirable for such sputter targets to have low oxygen contents. For manufacturing cost effectiveness, it may also be important for the targets to sputter with high deposition rates at low plasma impedances.
The magneto-optical recording medium industry is very competitive. Extensive amounts of research and development resources have been and are being expended in a continuous effort to better understand the relationship between sputter target materials and manufacturing processes on one hand and the desired properties of the resulting magneto-optical recording medium on the other. Even so, there is still a need for alternative materials and manufacturing processes for making sputter targets which enable higher quality MO films to be consistently produced. The present invention significantly contributes to this effort.